Liquid crystal display systems typically use backlights. Traditionally, the backlight produced constant and even light, with the liquid crystal cells controlling the brightness of the image. However, constant backlights have some disadvantages in high power consumption especially at high ambient light, heat generation and reduction in the dynamic range of the display. One solution for better control of the backlight replaces the constant backlight panel with an array of solid-state light emitters, such as light-emitting diodes (LEDs), with the number of LEDs being far less than the number of LCD elements. This allows for adjustment of the backlight according to the brightness in regions of the image, rather than the overall brightness of the entire image.
When using a backlight, the input image is typically downsampled to a resolution that corresponds to the LED array size. There are several methods that can be used to down sample the data. One method lowpass filters the data before downsampling and then adjusts that data to take into account the amount of light leaking from adjacent LED zones, where a zone consists of the area that is in front of the LED. Each zone represents the LCD elements/pixels closest to a particular LED, or group of LEDs, that are controlled together. To save driver cost and allow for a thinner panel a zone might consist of several LEDs that are controlled together so that they act like a single LED at a larger distance from the LCD panel.
Another method controls the LED value based on the maximum image data value for an LED zone. Another method might look at the histogram data of the input image associated with the zone. In any of the above approaches, the zone area might also be increased so that it overlaps with adjacent zones. Some systems may also apply a spatial or temporal weight to the data. These approaches represent just some of the ways of calculating the LED values.
However they are determined, once one has the LED values for the LED array, the system needs to adjust the input image pixels to achieve a desired image value. A typical desired image value is the input image value. The desired value results from the LED backlight illumination at a pixel multiplied by the transmittance of the pixel.
When the dynamic range of a display is increased, it may also be desirable to increase the dynamic range and/or adjust the look of the image to take advantage of the increase. In addition, because the frequency response of the LED resolution is much lower than the input image compromises might be required to reduce the level of artifacts. These compromises might result in an LED illumination too low to allow the reproduction of the original image. For example, it might require a pixel transmittance of greater than 100% which is impossible. In the current art, a value corresponding to a transmittance of greater than 100% requires either a soft clipping circuit or results in areas of the image with no detail.